Phlox, first brought into ornamental cultivation in the 1700s, is a large-flowered native perennial garden plant that has enjoyed centuries of popularity in Europe, but has not been extensively trialed in the United States, its country of origin—until now.

In a newly released report, Phlox for the Mid-Atlantic Region, Mt. Cuba Center’s research horticulturist details the results of three years of observation and testing of more than 94 selections of different species of phlox.

Read the full report here: https://mtcubacenter.org/trials/phlox-for-sun/

“Phlox has bold flowers. They’re in clusters as big as a softball, if not bigger,” says George Coombs, Mt. Cuba Center’s Research Manager. “They bloom at a time when not a lot of other plants are blooming, right in the heat of summer, and butterflies love them.”

Phlox can be purple, pink, white, and everything in-between. It suffers from powdery mildew, a fungal infestation of the foliage which strikes when the plant is stressed – in high heat or drought, and usually while flowering—casting a white, powdery look on the foliage. The condition usually will not be fatal to the plant, but it is unsightly for a cultivated garden.

“We wanted to test the plants’ resistance to powdery mildew, especially cultivars from European breeding programs,” Coombs says. “There are different disease pressures in Europe, so cultivars advertised as disease-resistant may not live up to that description.”

The Trial Garden at Mt. Cuba Center evaluates native plants and their related cultivars for their horticultural and ecological value. The goal of this research is to provide gardeners and the horticultural industry with information about superior plants for the mid-Atlantic region.

Coombs found the top-performing cultivars almost all came from U.S. cultivation, or were selections found in the wild. The top performer, Phlox paniculata ‘Jeana,’ was discovered growing on a riverside in Nashville, Tenn. While the plant was brought into cultivation for its strong resistance to powdery mildew, observers discovered it is highly attractive to butterflies.

“’Jeana’ was far and away the most popular for butterflies, especially eastern tiger swallowtails,” he says. “We did some studies to determine why that might be, and we found that its nectar is no different from other phlox.”

Mt. Cuba Center’s researchers used a team of volunteers to perform citizen science—called the Pollinator Watch Team. The team of fifteen volunteers conducted weekly observations of each plant in the trial to count the numbers of pollinators. During the two-year observation period, they reported 539 butterfly visits to Phlox paniculata ‘Jeana.’

Other top-performing phlox cultivars in the trial include: Phlox paniculata ‘Glamour Girl,’ P. paniculata ‘Delta Snow,’ Phlox x arendsii ‘Babyface,’ and P. paniculata ‘Lavelle.’

‘Jeana’ This cultivar stood out for its exceptionally mildew-free foliage. This trait carries through to the garden and is one of the main reasons ‘Jeana’ performed so well in the trial. This tall beauty also produces an impressive floral display from mid-July through early September. Interestingly, the individual flowers, or pips, are much smaller than any other garden phlox. However, that does not deter the butterflies that feed on its nectar. In fact, we found ‘Jeana’ attracted more butterflies than any other garden phlox in the entire trial. Blooms from mid-July through early September. Plants grow to 5 feet high and 4 feet wide.

'Delta Snow’ Part of the longstanding appeal of this cultivar is its exceptional resistance to powdery mildew. ‘Delta Snow’ has been recognized as one of the most disease resistant cultivars in several trial programs throughout the United States. Although its claim to fame is disease resistance, ‘Delta Snow’ also produces an incredible number of large inflorescences with white flowers accentuated by a bright lavender center. The weight of all the flowers did cause the stems to slightly angle outward; however, it wasn’t severe enough to reduce the plants overall appearance. It blooms from early July through late August, and it grows to 4 feet high by 40 inches wide.

‘Glamour Girl’ ‘Glamour Girl’ is a medium height cultivar (3 feet high) with stunning coral-pink flowers in midsummer. The large blooms begin in early July and last for nearly six weeks. In addition to its gorgeous flowers, ‘Glamour Girl’ also stands out for its vigorous and lush habit. Some powdery mildew was observed throughout the trial; however, it was never severe and did not cause much actual damage to the leaves. In a garden without hundreds of other phlox plants ‘Glamour Girl’ would likely be exceptionally clean.

‘Babyface’ This cultivar is thought to be a hybrid of P. paniculata and P. divaricata, which is typically denoted as P. × arendsii. Although this cultivar may have been derived from such a hybrid, it looks and behaves like P. paniculata. ‘Babyface’ does develop some powdery mildew, and frequently drops its lower leaves as a result. However, all of that can easily be forgiven thanks to its first-class floral display. The broadly pyramidal inflorescences are made up of small pink flowers with a darker pink center. The buds are also extremely attractive thanks to the dark calyx found at the base of each bud. This provides as much as two additional weeks of interest prior to the flowers opening. ‘Babyface’ also has a relatively late floral display which peaks in early August, a few weeks later than most cultivars. It grows to 32 inches high and wide.

‘Lavelle’ ‘Lavelle’ originated in the garden of Jeana Prewitt and is thought to be a seedling of the cultivar ‘Jeana.’ Although ‘Lavelle’ is not as disease free as ‘Jeana,’ it still boasts above-average resistance to powdery mildew. ‘Lavelle’ has white flowers with pale pink floral tubes that are held on 4-foot tall stems. The floral display is one of the longest in the trial, lasting from early July through late August. ‘Lavelle’ did suffer from being placed next to the sidewalk which reflected a lot of heat and greatly contributed to spider mite infestations. Despite these challenges, ‘Lavelle’ proved vigorous and resilient and would likely perform even better in a less stressful environment.